INCREASED MERCURY CONTROL IN A WET DEPURATOR THROUGH REDUCED OXIDATION AIR FLOW
DESCRIPTION OF THE INVENTION The subject matter of the present invention was developed under a research contract with the Department of North American Energy (DOE), Contract No. DE-FC22-94 PC94251, and under a contract of execution with the Office of Development of Ohio Coal (OCDO), Concession Agreement No. CDO / D-922-13. The governments of the United States and Ohio have certain rights over the invention. This is a continuation request in part for the pending North American Application Serial Number 09 / 282,483 filed on March 31, 1999 entitled INCREASED MERCURY CONTROL IN A HUMID DEPURATOR THROUGH REDUCED OXIDATION AIR FLOW. This patent application, serial number 09 / 282,483 is incorporated herein by reference. Unless stated otherwise, the definitions of the terms in series number 09 / 282,483 are valid for said description in the same way. The present invention relates in general to the field of industrial gas cleaning methods and apparatuses and, in particular, to a novel and useful method and apparatus for the removal of mercury from industrial gas generated by combustion which is treated with a wet scrubber . In recent years, the Department of North American Energy (DOE) and the US Environmental Protection Agency (EPA) have supported research to measure and control the emissions of hazardous air pollutants (HAPs) from useful heaters that are ignited by coal and plants of waste energy. The initial results of several research projects showed that emissions of heavy metals and volatile organic carbons (VOCs) are very low, except for mercury (Hg). Unlike most other metals, most mercury remains in the vapor phase and does not condense into volatile ash particles at temperatures typically used in electrostatic precipitators and fabric filters. Therefore, you can not collect and deposit a volatile ash like the other metals. For complicated matters, mercury can exist in its oxidized (Hg + 2) or elemental (Hg °) form and each is affected differently by the subsequent downstream contamination control equipment. In a conventional wet scrubber, Hg + 2 is relatively easy to control while HgO capture is difficult. The relative amount of each species seems to depend on several factors such as the type of fuel, the efficiency of the combustion heater, the type of particulate collector installed, and various other factors. With regard to the type of particular collector installed, it has been shown that an electrostatic precipitator (ESP), such as that used in most utility applications, affects the chemistry of the process and as a result the Hg becomes Hg ° inside a humid downstream scrubber, also commonly used in utility applications to reduce S02 emissions. The Hg is then emitted with the combustion gas. For experienced inventors, there is no prior related technique for controlling oxidation air flow rates in flue gas desulfurization (FGD) systems to improve mercury control. The present invention is based on the postulate that small concentrations of sulfur in the flue gas assist in the removal of mercury. The McDermott Technology, Inc. (MTI) test to assess mercury control in wet FGD systems indicates that there is a significant reduction in the effectiveness of mercury control when the wet scrubber is preceded by an electrostatic precipitator (ESP) against a filter of cloth. Likewise, elemental levels of mercury were increased in the wet scrubber when an ESP was used upstream, while the elemental levels of mercury were approximately the same when a cloth filter was used. It was concluded that mercury control is improved by the presence of "sulfur donation" such as hydrogen sulfide (H2S) by reaction with oxidized mercury (Hg + 2) to form mercuric sulfide (HgS). In addition, it was concluded that a portion of these sulfur donation species are destroyed in ESP by corona and ozone, and without these species for rapid conversion, oxidized mercury will be reduced to elemental mercury by reactions. Elemental mercury is insoluble in aqueous solutions and therefore leaves the wet scrubber with the combustion gas. Although sulfides are available in what are typically considered low concentrations, the mercury concentrations that enter the absorber typically range from 2 to 30 μg / dscm (micrograms / dry standard cubic meter) or that corresponds to only 0.2 to 3.3 parts per billion ( ppb). Therefore, even if some species are present only in the parts per million (ppm) scale, it is approximately three orders of magnitude higher in concentrations than mercury. Additional information supporting the hypotheses includes the observation during the ITN test that almost all of the mercury in the wet scrubber mixture was present in the solid fraction. This supports the hypothesis that the final form of mercury is HgS since the solubility product of HgS is 3xl0 ~ 52 and therefore it will precipitate out of the aqueous phase. Accordingly, one aspect of the present invention is to remove a method that uses a wet scrubber to receive and purify an industrial gas containing mercury with a wet scrubber mix, the wet scrubber has an absorber reaction tank that contains the scrubber mix wet to which oxidation air is supplied, and in particular the improvement comprises: reducing a supply of oxidation air to the reaction tank of the absorber to reduce the mercury content in the industrial gas leaving the wet scrubber. Another aspect of the present invention is to remove an apparatus that uses a wet scrubber to receive and purify an industrial gas containing mercury with a wet scrubber mixture, the wet scrubber has an absorber reaction tank that contains the wet scrubber mixture to which oxidation air is supplied, and in particular the improvement comprises: means for reducing oxidation air connected to the reaction tank of the absorber to reduce the supply of oxidation air to the reaction tank of the absorber to reduce the mercury content in the industrial gas that comes out of the wet scrubber. Yet another aspect of the present invention is to eliminate a method of using a wet scrubber that receives and scrubs an industrial gas containing mercury with a wet scrubber mixture, the wet scrubber has a reaction tank of the absorber containing the slurry mixture. humid scrubber to which oxidation air is supplied. Said method comprises: control of the oxidation air flow rate supplied in the wet scrubber mixture within the reaction tank of the absorber at a reduced speed relative to said oxidation air flow rate required to nominally achieve 100% oxidation of the wet scrubber mix. In this way, the wet scrubber mix used to purify the industrial gas contains a desired sulfite concentration sufficient to achieve the removal of greater mercury from industrial gas than that obtained when the wet scrubber mixture is supplied with sufficient oxidation air. to achieve nominally 100% oxidation thereof. Another aspect of the present invention is to eliminate a combination, with a wet scrubber that receives and rubs an industrial gas containing mercury with a wet scrubber mixture., the wet scrubber has a reaction tank of the absorber containing the wet scrubber mixture, and means to provide oxidation air to the wet scrubber mixture in the reaction tank of the absorber, oxidation air reduction means to reduce the Oxidation air velocity supplied in the wet scrubber mixture in the absorber reaction tank. The oxidation air reduction medium reduces the amount of oxidation air provided in the wet scrubber mixture by an amount sufficient to achieve the removal of greater mercury from industrial gas than that obtained when the wet scrubber is supplied with air and enough oxidation to nominally reach 100% oxidation of the wet scrubber mixture. Yet another aspect of the present invention is to eliminate a combination with a wet scrubber that receives and scrubs an industrial gas containing mercury with a wet scrubber mixture, the wet scrubber has an absorber reaction tank containing the wet scrubber mixture at which oxidation air is supplied; means for providing a controllable velocity of the oxidation air flow to the wet scrubber mixture in the absorber reaction tank; and means for controlling the flow rate of the oxidation air for the wet scrubber mixture in the reaction tank of the absorber in response to a sulfite concentration measured in the wet scrubber mixture. The control means is operative to control the oxidation air flow rate at a reduced speed relative to that required to nominally achieve 100% oxidation of the wet scrubber mixture to ensure that the wet scrubber mixture used to purify the gas industrial contains a selected non-zero sulphite concentration. In this way, the combination is able to achieve the removal of larger mercury from the previous industrial gas than that obtained when the wet scrubber is supplied with sufficient oxidation air to reach nominally 100% oxidation of the wet scrubber mixture. The various features of novelty characterizing the invention are pointed out with particularity in the appended claims to and form part of said description. For a better understanding of the invention, its operating advantages and specific objectives achieved by its uses, reference is made to the appended drawings and descriptive matter in which it illustrates a preferred embodiment of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of a wet scrubber and secondary systems used according to a first embodiment of the present invention; Figure 2 is a schematic illustration of a wet scrubber and secondary systems used according to a second embodiment of the present invention; Figure 3 is a graph illustrating how variations in air and oxidation provided to a wet scrubber affect the removal of mercury from the flue gas treated by the wet scrubber; and Figure 4 is a graph illustrating the percent oxidation as a function of sulfite concentration in millimoles / liter, in the wet scrubber mixture. With reference generally to the drawings, in which like reference numerals designate the same or similar functionally parts through the various drawings and to Figure 1 in particular, a first embodiment of the present invention is shown as it is applied to the installation. of a wet scrubber used to treat the combustion gas, generally referenced as facility 10. Obviously, although the method of the present invention is very likely to first find the commercial application for the removal of mercury from the combustion gases produced by the facilities of utility heater that subject combustion to fossil fuels, such as coal, any industrial process that uses a type of wet scrubber of absorber module to purify said combustion gases can be beneficial. Such procedures may include incineration plants, waste for power plants, or other industrial processes that generate gaseous products containing mercury. Thus, for convenience, the terms industrial gas, combustion gas, or simply gas will be used in the following description to refer to any gas in an industrial process and which will remove an objectionable component such as mercury. Additionally, for more detail of various aspects of said facilities 10, reference is made to the STEAM reader for its generation and use, 40th Ed. , Stultz and Kitto, Eds., Copyright © 1992 The Babcock & ilcox Company, particularly Chapter 35 - Control of Sulfur Dioxide, the text of which is incorporated herein by reference as fully set forth herein. Although the STEAM reference mentioned above contains a description of a form of wet scrubber produced by The Babcock &; Wilcox Company (B &) and which is applicable to the present invention, the present invention is not limited to such wet scrubber designs B & W. Those skilled in the art prefer that the principles of the present invention apply equally to other types of wet scrubber designs, available from other manufacturers. Generally, the present invention involves the provision of a supply of sulfur donation species to control mercury emissions in combustion or industrial gas 12 which is treated in a wet scrubber 14. In accordance with the present invention, it is concluded that the The above can be achieved by reducing the oxidation air supplied to another absorber reaction tank (ART) 16 which forms a lower portion of the wet scrubber 14. As a result, the mercury in the combustion gas 12 is likely to end up as mercuric sulfide , which is relatively stable as a solid and has the mercury form most commonly found in nature. The combustion gas 12 is typically generated by the combustion of fossil fuels and / or solid waste upstream (with respect to a direction of combustion gas flowing through the installation 10) equipment (not shown) and is transported to the wet scrubber 14 through the inlet pipe 18. In several installations, the combustion gas 12 is first conveyed through particulate removal means, schematically indicated at 20, to remove the ash and other particles from the flow gas 12 before the The combustion gas 12 provides the wet scrubber 14. As will be appreciated by those skilled in the art, the particulate removal means 20 may comprise a cloth filter, electrostatic precipitator, or similar devices suitable for the removal of a desired amount of particles. of the combustion gas 12. After treatment in the wet scrubber 14, the combustion gas 22 leaves the scrubber 14 through the is from the outlet of the wet scrubber 24. Oxidation air 26 provided by the compressor or fancy means 28 is conveyed to ART 16 by oxidation air supply lines 30, and 32 to promote the oxidation of sulfite species in a wet scrubber mixture 34 contained in ART 16 to maximize the formation of calcium sulfate dihydrate (CaS04 • 2H? 0); that is, plaster. The conventional wet scrubbers 14 are currently operated / controlled in what is called a "continuous feeder" control scheme, ie, the appropriate level of S02 removal is designed in a simple way, or more precisely on the design, in the flue gas desulfurization system. For example, in the wet scrubbers 14 that use forced oxidation, the compressors that provide the oxidation air in the wet scrubber 14 are designed to operate in an extended manner, so that they will provide more sufficient oxidation air based on the conditions of the design. Typically, due to the poor reliability of the measurement, S02 measurements are not ascertained to form an input signal to a control system. In other words, the "feedback" control is typically not used. In this way, in contrast to the conventional method of operation of said facilities where sufficient oxidation air is provided in the wet scrubber mixture 34 to ensure the almost complete oxidation of the sulfite species to produce gypsum, the present invention reduces intentionally the flow rate of the oxidation air 26 provided to ART 16 at the levels that will not produce approximately or essentially 100% oxidation of the sulfites in gypsum. Therefore, the current application will use the term "nominally 100% oxidation" to refer to the degree of forced oxidation obtained under conventional wet scrubber operating procedures. The operation of the wet scrubber installation 10, in this way, the reduced oxidation air flow rates 26 achieve less than nominally 100% oxidation of the wet scrubber mixture 34. As a result of the non-zero concentrations or sulfite levels present in the wet scrubber mixture 34 , expressed in ppm levels (parts per million) but still this relatively low concentration of sulfite is an amount sufficient to allow the formation of mercuric sulfide, HgS which can be collected by the wet scrubber 14. It is recognized that the systems of forced oxidation of purification with water that produce essentially all the plaster, some sulfites can be detected. In other words, one can find "non-zero" concentrations of sulfites in gypsum samples obtained from systems that are considered to be operational at "100%" nominal oxidation conditions. In this way it is important to recognize that, in accordance with the present invention, the reduction of the amount of oxidation air 26 provided to the wet scrubber will gradually lead to greater control of mercury emissions from the wet scrubber 14. Typical wet scrubber installations 10 employ more than one compressor 28 to supply the forced oxidation air 26 in the wet scrubber module 14. In accordance with the present invention, by reducing the oxidation air 26 one of the compressors 28 can be turned off thereby reducing the amount of oxidation air 26 that is being supplied. Alternatively, and in a different situation where only one compressor 28 is provided, said compressor 28 would probably stop operating at its designed speed, but a portion of the oxidation air 26 may be mixed prior to its introduction into the wet scrubber module 14. , thereby reducing the oxidation air velocity 26 provided in the wet scrubber module 14. Accordingly, the present invention reduces oxidation air flow rates 26 to ART 16, below the amounts typically used. during operation to a forced installation wet scrubber installation 10, in order to reach the point at which the sulfites are measurable in the wet scrubber mixture 34, preferably levels greater than about 0.3 millimoles / liter. One way to identify such point of operation or condition (ie, slightly below 100% oxidation for gypsum) is by reducing the flow rate and oxidation air 26, and at the same time, to monitor the concentration of sulfites in the wet scrubber mixture 34 to determine the point where the sulfites begin to appear in the liquid phase wet scrubber 34 mixture in low but measurable amounts. In other words, the concentration of sulfite in the wet scrubber mixture 34 can be used as an indicator of the extent of oxidation, and therefore, the point at which the sulfide donation species are likely to be present, can determine. As described above infra, another aspect of the present invention involves the use of a measurement of sulfite concentration in the wet scrubber mix 34 as an input parameter for a control system that can be used to control the speed of the introduction of oxidation air 26 in the ART 16. During the operation of the wet scrubber 14, the recirculation pumps 33 pump and recirculate the wet scrubber mixture 34 of the ART 16 through the pipes 35 and in the located spray crates 37 in an upper portion of the wet scrubber 14. The wet scrubber mixture 34 is sprayed on the current counter in the flue gas 12 where it absorbs the SO2. The wet scrubber mixture 34 falls through several devices and is re-drained to the ART 16. A small fraction of the wet scrubber mixture 34 of the ART 16 is recirculated by the pumps 33 and diverted to a water treatment system. dehydration 36, such as hydroclone, connected to the ART 16. The wet scrubber mixture 34 typically contains 15% suspended solids, and hydroclone 36 concentrates to the wet scrubber mixture 34. Hydroclone 36 receives the wet scrubber 34 mixture ( also differentiated as a purged gypsum mixture containing gypsum and water) by the gypsum purge line 38. The inflow portion or stream of hydroclone 36 is concentrated to approximately 25% solids and removed from the system by the infra-flow line 42. The overflow portion or stream of hydroclone 36 contains about 4% solids and is sent back to ART 16 via the overflow line 40. In typical wet scrubber installations 10, nominally 100% oxidation of the Calcium sulfites in calcium sulfate hydrate (CaS04-2H20) or gypsum is required to achieve "gypsum board quality" gypsum. According to the present invention, if less than nominally 100% oxidation is desired to achieve the desired mercury control and gypsum quality gypsum control levels if desired, another aspect of the present invention involves the provision of an optional system, generally designated as 44, which provides for the secondary oxidation of the inflow hydroplane stream transported through the inflow line 42. More particularly, the inflow line 42 transports the purged gypsum mixture 34 to a secondary oxidation tank 46 and an associated mixer 48. The secondary oxidation system 44 thus allows the optional secondary oxidation of said portion of the purged gypsum mixture 34 of hydroclone 36 to convert to the last fraction of the calcium sulfites. in plaster. Since only a small portion of the purged gypsum mixture 34 needs to be oxidized therein, the size and cost of the equipment (tank 46 and mixing means 48) in the secondary oxidation system 44 will be minimal. The foregoing is true for line 50 and associated pump means 52 used to convey the final gypsum product stream 54 to the downstream processing equipment (not shown). For experienced inventors, secondary oxidation is not used by others to complete the last oxidation fraction, or to facilitate mercury control. The reduction of the amount of oxidation air used in the ART 16 can be done without the existing equipment and without the introduction of any additive to the system. If the purity of the gypsum product stream 54 falls below the system specifications for a given application, the equipment and method identified above for secondary oxidation can be used to give purity to gypsum product stream 54 of new with specifications. The requirements of the oxidation air 26 for the secondary oxidation system 44 (provided along the air line 56 to the tank 46) will be equally minimal since the final oxidation is carried out only in a small fraction of the volume original at a time. It should be noted that several wet scrubber installations 10 include a puff tank down the absorber (not shown), and that said tank can be used as a secondary oxidation system tank 46. This technique was evaluated in the pilot test by MTI. . For several coals, mercury measurements were carried out in the wet scrubber 14 while using the standard and reduced oxidation air flow rates. The results consistently showed that improved mercury capture and increases in elemental mercury in the scrubber were avoided when the reduced oxidation air flow rates were tested as shown in Figure 3. In Figure 3, the legends " Input WS "and" Outputs WS "refer to the conditions at the inlet and outlet of the wet scrubber 14, respectively. Figure 3 shows that with the conventional 26 oxidation air flow rates (the left end pair of the bar graphs in Figure 3 labeled as "Base Line Ox Air"), 46% of the total mercury removal was achieved with a significant increase in the mercury output percentage of the wet scrubber 14 as elemental mercury (Hg °). When the oxidation air flow velocity 26 was slightly reduced (the average pair of bar graphs in Figure 3 labeled as ("Ox Medium Air"), 57% of the total mercury removal was achieved. the percentage of the mercury output of the wet scrubber 14 as elemental mercury (Hg °) was observed, the percentage increase was lower than that of the Ox Ox Baseline case, and finally, when the oxidation air flow velocity 26 was reduced enough to observe the sulfites in the wet scrubber mix 34 (the right end pair of the bar graphs in Figure 3 labeled "Low Ox air"), 80% of the total mercury removal was achieved, and the percentage of the mercury output from the wet scrubber 14 as elemental mercury (Hg °) remained virtually unchanged.Therefore, said test results indicated that the control of the oxidation air flow rate to a minimum value required for the formation of high purity gypsum is an effective means of improved mercury control in FGD systems. The mercury in the combustion gas 12 ends up as mercuric sulfide, which is relatively stable as a solid and is found in the mercury form most commonly found in nature. It should be noted that the high conversion of calcium sulphite to calcium sulphate (gypsum) is important in order to minimize mixing in the wet scrubber 14 to ensure high purity gypsum for cases where quality gypsum is required. plasterboard. However, tests showed that significant improvements in mercury control are obtained with the presence of only small amounts of calcium sulfite. For applications where gypsum is embedded in the soil, gypsum purity is not a problem and the oxidation air flow rates 26 can be controlled to the extent necessary to control mercury emissions, evidently limited by the extent of the mixture that can be tolerated in the wet scrubber 14. Mixing is a term of the art that refers to deactivation of the absorbent particles (typically lime) through encapsulation by sulfite crystals. Figure 4 is a graph illustrating the result of calculations of the percent oxidation as a function of sulfite concentration, in millimoles / liter in the wet scrubber mixture. For the purposes of the present invention, in order to maximize the mercury control of oxidation air flow, it must be controlled or regulated to achieve sulfite concentrations ranging from about 0.3 to about 20.0 millimoles / liter. These sulfite concentrations correspond, respectively, to a percent oxidation scale of approximately less than 100% oxidation around 98% oxidation. It is anticipated that the oxidation of said wet scrubber, the forced oxidation systems may operate so that the oxidation percentage remains above 98% of the oxidation level for two reasons. The main reason is that oxidation levels below 98% do not provide additional advantages in the control of mercury emissions. The second reason, as described above, is that at oxidation levels below 98% the wet scrubber systems enter an operating mode where the mixing phenomena are likely to become a major concern. For cases where plasterboard quality gypsum is required, the invention allows the optional secondary oxidation of the purged gypsum mixture 34 to convert the last fraction of calcium sulphites into gypsum. With reference to Figure 2, another aspect of the present invention is shown., converted to a method and apparatus using a sulfite concentration measurement in the wet scrubber mixture 34 as an input parameter to a control system for controlling the rate of introduction of the oxidation air 26 in the ART 16. For For greater clarity, the installation of the wet scrubber in Figure 2 is designated 100. Sulfite concentration measurements can be made in wet scrubber mix 34 that is recirculated in lines 35, in the wet scrubber mix 34 in the ART 16 itself, or at another location, such as the hydro-flow overflow stream conveyed by the overflow line 40. In this last place, however it should be noted that some low concentration of sulfites may be present in said stream during normal operation and this fact must be taken into consideration when calibrating the system. More particularly, said control system preferably comprises control means 60, advantageously based on a microprocessor, to receive the sulfite concentration signals indicating the concentration of sulfite in the wet scrubber mixture 34. One or more sensor means of concentration of Sulfite 62 can be provided for that purpose, and their respective signals can be transported through line 64 to controller means 60. Controller means 60 can also be provided with input means for receiving the concentration point attachment values of sulfite 66 from the human operator or other control device. The controlling means 60 operates to compare the sulfite concentrations measured or monitored against the values of the fixation point 66 and, as a result of said comparison, produces a control signal emitted through the line 68 whose control means for controlling the speed wherein the oxidation air 26 is provided to ART 16. As illustrated in Figure 2, a form of the means for controlling the oxidation air velocity 26 comprises an oxidation air control valve means 70 located at line 32 for controlling the amount of oxidation air 26 provided to ART 16. Control means 60 can be programmed to compare monitored sulphite concentrations to fix point values 66 at desired intervals determined empirically by variability of concentrations of sulfite with time
(adaptation control), or simply at predetermined fixed intervals. Alternatively, if said type of control feedback is determined to be necessary or complex in its entirety, the controlling means 60 may be programmed to merely set a control signal fixed to the medium for controlling the oxidation air flow 70. The deployment means 72 and the data storage means 74 can also be connected to the controller means 60, to provide a display of fixation point or monitored sulfite concentration values to be observed by a human operator, or to record the data concerning them respectively. In addition, the present invention contemplates that the sulfite concentration sensor means 62 can be supplied with batch and physical samples at the locations where said sensor means 62 may have been employed, can be taken and analyzed at a remote location, such as a laboratory. . In such a case, a sophisticated control system may be required and substantially the fixed oxidation air flow rates 26 may be established based on the set points established in the field. Although a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention can be represented otherwise without departing from said principles. It will be particularly appreciated that the present invention can be applied to repair, modify or replace existing wet scrubber installations employing forced oxidation, as well as new wet scrubber installations. In certain embodiments of the present invention, various features of the invention can be employed without corresponding use of other features. All these modalities, however, fall within the scope and equivalents of the following claims.